192 CHEMICAL TECHNIQUES 



The quartz stand (B) supports these fibers. Fine quartz loops in the 

 ends of the beam hold quartz hooks (E) from which the aluminum 

 foil pans (D) are suspended. The two arms of the beam need not 

 be of exactly the same length. The feet (G) of the standard are 

 sealed to the floor of a balance case with DeKhotinsky cement. It is 

 convenient to employ the usual mechanism in analytical balances 

 that lifts the beam when loads are added to, or removed from, the 

 pans. Electrostatic shielding is achieved by lining the inside of the 

 balance case with metal foil in which windows (H) are cut. In addi- 

 tion, the members of the balance are metalized by coating them with 

 a 5% solution of chloroplatinic acid in alcohol and, after drying, 

 heating with a "cool" flame to effect conversion to metaUic platinum. 

 Particular care is required to avoid overheating the fine quartz sus- 

 pensions. A strip of aluminum foil is used to ground the instrument 

 to the case. 



Fig. 75. Quartz torsion balance. 

 From Lowry (1944) 



After the case has been closed for at least 1 min., measurement of 

 the displacement of one end of the beam produced by the load placed 

 on a pan is made with a cathetometer (F) reading to ±:0.01 mm. In a 

 particular balance constructed by Lowry, a load of 10.8 ^ug. pro- 

 duced a displacement of 1.00 mm. The cathetometer may be focused 

 on any convenient landmark on one end of the beam. An illuminated 

 piece of white paper outside the opposite end of the balance case 

 furnishes a background that facilitates the measurement. 



Calibration of the balance is carried out by cutting 5-10 cm. of 

 fine wire, weighing 1.5-2 mg., into ten nearly equal lengths, weigh- 



